The objective of this research program is to study the mechanism of radiation resistance in microorganisms. A series of Salmonella typhimurium mutants with increasing resistance to both UV and ionizing radiation have been developed and partially characterized. They are readily identified as Salmonella since they retain most of the normal phenotypic characteristics, even though some mutants are as radiation resistant as Micrococcus radiodurans. This study is to investigate the mechanisms by which these organisms have become radiation resistant and to establish the biochemical and physical role of DNA metabolism, as a determinant of radiation resistance in these mutants. With these mutants a second objective is to develop an in vitro system of repairing damage in DNA. The third objective is to identify, map and to characterize the role of genes affecting resistance to radiation of Salmonella typhimurium.